For example, DC brushless spindle motors are used in several applications, amongst others in optical and magnetic disc drives to rotate the storage medium. Other application fields are, for example, VCRs, shavers, fans, blowers and robotics. Braking these motors can be done electronically by driving the motor in such a way that the motor current reverses polarity relative to forward rotation. Since during deceleration the motor also acts as a generator, braking regimes are possible where a net current is delivered back to the power supply. Mechanical energy present in the inertia of the rotor is recuperated to electrical energy for the supply battery or capacitor. Using this regenerative braking regime yields a green and power-efficient motor drive system.
One known problem of regenerative braking is the possibility of over-voltages on the power supply when the power supply cannot deal with negative currents. Usually this is the case since asymmetric linear or switched-mode controllers are used to regulate the power supply voltage. Consequently, the power supply decoupling capacitor which is usually provided has to deal with these negative motor-induced back currents and given the large amount of energy in the motor dangerous over-voltages can occur. A known method to overcome these over-voltages is to use a Zener diode protection or an on-chip active clamp. This involves extra hardware cost since these devices need to be able to deal with considerable power dissipation (e.g. 4 W for optical disc drives). To overcome these problems it is for example known from U.S. Pat. No. 6,949,906 B2, which discloses a method and a controller of the type mentioned at the beginning, to either short circuit or set into a high impedance mode the windings of a DC motor for a predetermined time interval, if a dangerous condition is detected during the deceleration process of the DC motor, for example an over-voltage at the power supply output. If the windings are short circuited, energy is dissipated because of friction and because the back-EMF of the motor results in current through the winding resistance. If the windings of the motor are set into a high impedance mode by opening the winding connections, energy is dissipated by friction only. However, short circuiting or setting into a high impedance mode the windings of the motor for a predetermined time interval may lead to a waste of energy, if the predetermined time interval is longer than the duration of the dangerous condition. This may often be the case because the predetermined time interval has to be selected long enough to ensure that the dangerous condition is no longer present and will not occur again, particularly not in an even worse form, if the normal deceleration process is resumed.
It is the object of the invention to further develop the methods and controllers of the type mentioned at the beginning such that the waste of energy discussed above is avoided.